Temperature-dependent luminescence spectroscopic investigations of uranyl() complexation with the halides F and Cl

In the present study we have investigated the complexation of uranyl( vi ) with chloride and fluoride using luminescence spectroscopy (TRLFS, time-resolved laser-induced fluorescence spectroscopy). At 25 °C (298.15 K), in the presence of 0-0.175 M fluoride, the first single-component emission spectra for all four uranyl( vi )-fluoride complexes, i.e. UO 2 F + , UO 2 F 2 , UO 2 F 3 − , and UO 2 F 4 2− could be extracted. Based on the aqueous speciation derived from the TRLFS data, log K* values at I = 1 M were calculated for all these complexes and extrapolated to infinite dilution using the SIT approach. In the case of chloride, however, quenching of the uranyl( vi )-luminescence hampered the experiments. Thus, uranyl( vi )-complexation was studied with TRLFS at liquid nitrogen temperatures. Samples were prepared at 25 °C (298.15 K) with chloride concentrations ranging from 0 to 1.0 M followed by instantaneous freezing and subsequent luminescence spectroscopic measurements at −120 °C (153.15 K). This allowed for the determination of the first luminescence spectra for the UO 2 Cl + complex with the TRLFS method. The chloride quench reaction was further studied in the temperature range 1-45 °C (274.15-318.15 K) using Stern-Volmer analysis. By applying the Arrhenius and the Eyring equations we obtained the first thermodynamic parameters for the dynamic quench process, i.e. the activation energy ( E a = 55.0 ± 12.9 kJ mol −1 ), enthalpy (Δ H ‡ = 52.5 ± 13.0 kJ mol −1 ), and entropy (Δ S ‡ = 103.9 ± 42.8 J mol −1 K −1 ). Uranyl( vi ) complexation with fluoride and chloride was investigated with luminescence spectroscopy, and the strong quenching by chloride was overcome by freezing.